DAHP (3-deoxy-D-arabino-heptulosonic acid 7-phosphate) synthesase E.C. 4.1.2.15, the first enzyme of the multibranched pathway that leads to the synthesis of the three aromatic amino acids phenylalanine, tyrosine, and tryptophan and to the synthesis of several vitamins and the iron chelating compound enterochelin appears in Escherichia coli in three isoenzymic forms, each feedback inhibited by one of the aromatic amino acids. We have purified to homogeneity two of the isoenzymes, and we have started the physico-chemical characterization of these proteins. Attempts will be made to purify the third isoenzyme. Extensive kinetic analysis of the tyrosine sensitive isoenzyme suggested two binding sites for the feedback-inhibitor. Fluorescence spectroscopy will be employed to substantiate these finds, to determine the binding constants and to initiate studies on conformation changes caused by the inhibitor. Kinetic analysis of the phenylalanine sensitive isoenzyme will be performed. Also, primary structural analysis of the two purified isoenzymes will be undertaken. BIBLIOGRAPHIC REFERENCE: Schoner, R., and Herrmann, K.M. "3-Deoxy-D-Arabino-Heptulosonate 7-Phosphate Synthase. Purification, Properties, and Kinetics of the Tyrosine-Sensitive Isoenzyme from Escherichia coli", J. Biol. Chem. 251, 5440-5447 (1976).